1. Field of the Invention
The present invention relates to a method and apparatus for improving the efficiency of an acousto-optic deflector or Bragg cell by successively returning the undeflected light in the zero order beam back through the acoustic wave within the cell. This allows successive acousto-optic interactions to take place for increasing the overall deflection efficiency.
2. Description of Prior Art
The term acousto-optics (A/O) refers to an interaction of light and sound. Typically an RF input signal is first transformed into an acoustic wave in a suitable crystal material, such as lithium niobate. Variation in index of refraction due to the propagation of the acoustic wave within the crystal can be then used to deflect a beam of light, usually monochromatic. This process is the equivalent of the better known Bragg diffraction of X-rays from the planes of a crystal lattice; for this reason the device is called a Bragg deflector or Bragg cell as well as an acousto-optic deflector or modulator. The angular deflection of the optical beam is proportional to the frequency of the original RF input signal. As the process is linear, multiple simultaneous RF input signals yield multiple simultaneous beam deflections corresponding to the distinct input frequencies with the intensity of the individual deflected beams being proportional to the power of the original RF input signal.
Acousto-optics has been used for a variety of applications where light must be modulated or deflected. An important application is the use of acousto-optics for wideband receiving systems. The acousto-optic phenomenon occurs over a substantial bandwidth, 1 GHz with existing devices, so that the frequency content of an unknown signal environment can be resolved by measuring the angle of deflection corresponding to each signal in the environment. Thus, the entire signal environment may be viewed simultaneously by a device that acts like a channelized receiver.
One major problem with such wideband A/O receiver systems has not yet been resolved; dynamic ranges of experimental systems are disappointingly small, on the order of 30 dB for 100 nanoseconds response time. For this technology to be applicable to a practical receiver system design, improvements in dynamic range will be required. A number of factors contribute to this problem. Wideband Bragg cells are characteristically poor deflectors of light, deflecting typically less than 1% of the available optical power. The maximum deflected optical power is limited by Bragg cell material properties, transducer power handling capability, and available laser power. The deflected light output from a typical wideband Bragg cell has a two-tone spurious response free dynamic range of about 50 dB, for two signals at maximum amplitude. A dynamic range of even 40 dB cannot normally be realized in a system, however, due to limitations associated with sensing the optical signals. Photodiode sensitivity is limited by thermal noise of diode resistance. As video bandwidth is increased to guarantee acquisition of short (100-500 nanosecond) RF pulses, the sensitivity of the diode decreases. For example, typical operating conditions of the combination oi a 1 GHz lithium niobate Bragg cell and a 3 MHz video bandwidth PIN photodiode sensor in combination limit the dynamic range to only 30 dB.
Several approaches may yield enhanced deflection efficiency for wideband Bragg cell systems. Increased transducer power handling capabilities are possible, and increased deflection efficiencies can be achieved through the use of multiple transducers. Acoustic beam steering, to optimize the interaction region of light and sound, is a further approach which may be utilized with multiple transducer designs to yield increased deflection efficiency. Each of these proposed approaches has major drawbacks, and the gains to be expected from them are relatively small.
A need therefore continues to exist for a wideband (greater than 1 GHz bandwidth) A/O system having a deflection efficiency greater than 1% per watt.
Prior art devices which incorporate electro-optic modulators have been used in surface acoustic wave (SAW) systems to control the wave. The electro-optic modulators operate to deflect a beam of electromagnetic energy by the application of an electric field to the crystaline material of the modulator. These devices use a multiplicity of passes of the beam of light to increase beam deflection which is inherently small in such devices, see U.S. Pat. No. 3,492,063, by Tzuo-Chang Lee; or to create interference to modulate beam intensity, see U.S. Pat. No. 3,923,380, by Shuzo Hattori et al. and U.S. Pat. No. 3,813,142, by Carl F. Buhrer.
None of the prior art devices known address the problem of increasing the intensity of the deflected first order beam from an acousto-optic deflector.